Interdiffusion behavior of Al-rich oxidation resistant coatings on ferritic-martensitic alloys

Interdiffusion of thin Al-rich coatings synthesized by chemical vapor deposition (CVD) and pack cementation on 9Cr ferritic-martensitic alloys was investigated in the temperature range of 650-700°C. The compositional changes after long-term exposures in laboratory air and air + 10 vol% H ₂O were examined experimentally. Interdiffusion was modeled by a modified coating oxidation and substrate interdiffusion model (COSIM) program. The modification enabled the program to directly input the concentration profiles of the as-deposited coating determined by electron probe microanalysis (EPMA). Reasonable agreement was achieved between the simulated and experimental Al profiles after exposures. The model was also applied to predict coating lifetime at 650-700°C based on a minimum Al content (C_b) required at the coating surface to re-form protective oxide scale. In addition to a C_b value established from the failure of a thin CVD coating at 700°C, values reported for slurry aluminide coatings were also included in lifetime predictions.